Citing a "clear cultural shift," Alaska's senior US lawmaker on Tuesday became the third Senate Republican to openly back marriage equality, as the US Supreme Court prepares to rule on landmark gay rights cases.

The announcement by Sen. Lisa Murkowski brings to 54 the number of senators, including all but three of the Democrats in the 100-seat chamber, to openly support gay marriage.

Republican Senators Rob Portman and Mark Kirk were the first to buck their party platform when they announced their support earlier this year.

Senate Democrat Frank Lautenberg was a supporter, but he died early this month at age 89 and was replaced by a Republican who has not clarified his position on the issue.

Murkowski supported a 1998 constitutional amendment defining marriage as between a man and a woman, but said on her website Tuesday that "my thinking has evolved as America has witnessed a clear cultural shift.

"Fifteen years after that vote, I find that when one looks closer at the issue, you quickly realize that same sex unions or civil marriages are consistent with the independent mindset of our state, and they deserve a hands-off approach from our federal policies."

Twelve states have legalized same sex marriage, including three states in a two-week span in May, but constitutional bans against gay marriage remain in place in 31 of the 50 US states.

The Supreme Court is expected to rule this month on whether or not the Defense of Marriage Act, under which the federal government recognizes only heterosexual marriages, is constitutional.